1. Field of the Invention
The present invention relates to isolated pressure transmitters which are connected directly to an industry standard union.
2. Description of the Prior Art.
It is an industry standard practice to use one or more flange adapter unions for removably coupling process piping to isolated pressure transmitters. The flange adapter union is a coupling device which connects a process pipe carrying a pressurized fluid to a flanged surface on a pressure transmitter. The flange adapter union typically has a central threaded fitting at a first surface facing in a first direction for mechanically attaching the process pipe to the flange adapter union and for sealing the process pipe to the flange adapter union. The flange adapter union has a second surface facing in a second direction opposite the first direction adapted to couple to a massive flange on the pressure transmitter. The flange adapter union is removably attached to the flanged surface of the transmitter with a pair of bolts which also serve to compress a seal around a central fluid opening on the surface of the flange adapter union. The flange adapter union allows the transmitter to be coupled to a threaded pipe without rotating either the pipe or the transmitter to engage the pipe threads. The fluid opening on the second surface of the flange adapter union is small, and the spacing between the bolts is restricted, and hence a massive intermediate flange is used to couple the fluid from the small restricted opening on the flange adapter union to a larger isolator diaphragm in the transmitter.
The current standard adapter unions each have a pair of mounting bolts spaced on industry standard 15/8" bolt centers, and which are centered on a central pressure port that delivers fluid to the transmitter. The pressure port is normally threaded for a pipe connection, and the union must be rotated or screwed onto the pipe for connection to the input pressure source. Alternately, a three-valve manifold comprising another expensive, relatively heavy intermediate member having the same industry standard bolt mounting configuration on both sides has been disposed between the unions and the transmitter's flange.
In prior art transmitters, the commonly used isolator diaphragms have been too large to fit within the bolt mounting patterns of the flange adapter unions, and hence massive flanges as shown in U.S. Pat. No. 4,466,290 have been used to route fluid from the flange surface mating with the union to a flange surface that mates with a surface on the transmitter which is large enough to accomodate the large isolator diaphragms. The isolator diaphragms act through an incompressible fluid to couple pressure to a pressure sensor in the transmitter.
Prior art transmitters require large isolators because the pressure sensors require a relatively large volume of fluid behind the isolator diaphragms and the fill fluid expands as temperature increases. The isolator diaphragm has to be large so that it is sufficiently compliant to accomodate changes in volume without introducing a pressure measurement error by changing the pressure on the sensor itself. Also, prior art sensors required a relatively large displacement of fill fluid for actuation, so full scale deflection of the sensor needed a large cOmpliant isolation diaphragm to move the necessary amount of isolating fluid to provide the displacement.
Thus, while the standard adapter unions are widely used, they have had to be separately bolted onto flange assemblies, with the flange assembly then bolted to the transmitter, increasing the number of seals that are necessary, the number of bolts that are necessary, and also increasing the weight of the overall transmitter assembly. The large number and size of the seals contribute to leaks and limit the reliability of the transmitter. The larqe area of the flange exposed to the pressurized fluid is subject to a correspondingly large force which is transferred to the flange bolts creating a potential for breakage.